Radio communication devices, such as portable telephones, require a stable operating frequency provided by a local oscillator. Typically, the frequency of a local oscillator is generated by a voltage controlled oscillator (VCO). The operating frequency of voltage controlled oscillators used in these portable telephones is strongly dependent on parts tolerances and manufacturing techniques.
In particular, the etching variations of striplines defined on circuit boards of VCOs can cause large frequency shifts. Some prior art methods to adjust the operating frequency of VCOs have included; deforming coils, using trimmable capacitors and using variable inductors. These methods have the disadvantage of having the potential to change frequency in the field, either through a mechanical shock or through intentional physical adjustment. In addition, these methods are used before the VCO is sealed. Therefore, the frequency of the VCO may change after tuning due to the sealing process itself.
Other more permanent prior art methods to adjust the operating frequency of VCOs have used cutting techniques to remove conductive material from the circuit traces or specially provided large area capacitors designed to be mechanically trimmed. The actual cutting action was provided by laser trimming, sand abrasion, or mechanically by using a Dremel.TM. tool, for example. However, these methods are used before the VCO is sealed. Therefore, although these methods are more permanent, the frequency of the VCO may change due to the sealing process itself, as stated above.
There is a need for a VCO tuning method that is permanent and can be performed after final assembly of the VCO. There is also a need for a tuning method that does not destroy the integrity of the sealed VCO or introduce contaminants on top of the circuit board. In addition, there is a need for a tuning method that can be applied to a single circuit board being operable for VCOs having widely different frequencies.